Torque measuring apparatuses are known which are of the type having a pair of magnetically anisotropic portions formed on the periphery of the torque transmission shaft such that a change in the magnetic permeability of the magnetically anisotropic portions, when torque is applied to the shaft, is detected by a pair of sensing coils disposed adjacent the magnetically anisotropic portions, so that the magnitude of the torque acting on the shaft is converted into an electric signal on the basis of the difference between the detection signals from the sensing coils. Typically, torque measuring apparatuses of this sort have been proposed in, for example, Japanese Patent Application Laying-Open Publication No. 64-29723 and No. 63-33634.
In such torque measuring apparatus, sensor means comprises a shaft portion having magnetically anisotropic portions formed thereon, and other elements such as sensing coils and exciting coils. When a temperature change occurs in the sensor means, the magnetic and electrical characteristics of the sensor means are subject to changes. As a consequence, the torque sensing characteristics of the sensor means undergo considerable changes which in turn unfavorably affect the sensing accuracy of the sensor means. This poses a problem when torque is to be measured with respect to such kinds of equipment as industrial machinery, motors, engines, and automobiles, in which comparatively high temperatures are prevalent when they are in operation.
In the foregoing Japanese laying-open publication Nos. 64-29723 and 63-33634, therefore, it is arranged that the sensor means includes built-in temperature sensing elements, such as thermistors or thermo-sensitive resistances, or elements are provided for detecting the alternating exciting currents of the exciting coils thereby to detect the temperature at the sensor means, so that torque detection signals are temperature-compensated on the basis of the detection signals from such elements.
However, the provision of built-in temperature sensing elements in the sensor means results in increased number of parts, including cable lines required for transmission of temperature detection signals from the elements. This makes the construction and assembly of the sensor means very much complicated as a whole. Another problem is that variations in the inherent properties of the elements may result in variations in temperature characteristics.
Where the apparatus is adapted for detection of the alternating exciting currents of the exciting coils, the sum of changes due to temperature in the magnetic and electrical characteristics of individual components, such as shaft, coils, shields, and casing, which define a coil impedance is utilized to give a temperature detection signal. As such, characteristic variations, if any, of the components per se or as caused during assembly operation thereof may unfavorably affect the temperature detection accuracy of the sensor means. Another problem is that when torque is applied on the shaft, the exciting current is likely to fluctuate to some extent, because the construction and material of the sensor means cannot be of magnetically and electrically complete symmetry thereacross even if the apparatus is of the differential detection type. Moreover, it must be noted that the higher the temperature, the lower is the magnetic permeability .mu. of the sensor components, while on the other hand the higher the temperature, the higher is the resistivity .rho. of the sensor components. Also, it must be noted that the lower the magnetic permeability .mu., the lower is the impedance of the exciting coils, whereas the higher the resistivity .rho., the higher is the impedance of the exciting coils. Therefore, changes due to temperature of the impedance of the exciting coils eventually become smaller as changes due to temperature of both the magnetic permeability .mu. and the resistivity .rho. negate each other. Accordingly, changes of the exciting currents per se due to temperature changes become smaller and this necessitates a sensing circuit of higher sensitivity and higher detection accuracy. As such, no sufficient temperature compensation can be obtained, with an added disadvantage that the cost of the apparatus is unacceptably high.